I. Field of the Present Invention
This invention relates generally to an apparatus for controlling the speed of a motor and more specifically, this invention relates to an apparatus which inhibits the application of high torque when a motor operating at high speed in a first direction is switched to run in reverse direction.
II. Description of the Prior Art
In certain lift trucks, particularly in those lift trucks known as "walking trucks", the speed is controlled by varying the resistance in the DC motor circuit. Such lift trucks have three discrete speeds: first, second and third, depending on the series resistance to the DC motor.
When switching from forward to reverse direction is desired, the operator will often attempt to accomplish such a result by "plugging" the motor. "Plugging" commonly refers to reversing the direction of current in the field of a motor while it is still operating in the opposite direction. When the operator of such walking trucks shifts from the highest forward speed to reverse, a very high torque is imposed upon the unit. Such high torques have been known to cause loads carried by the trucks to spill and have caused the operators to fall from the truck itself thus causing injuries.
In order to avoid the high torque imposed upon a series resistor controlled DC motor type truck upon shifting from high speed forward directly to reverse, some technique must be employed to control deceleration of the DC motor armature from its velocity in the forward direction to zero velocity and thereafter to control acceleration of the DC motor in the opposite, or reverse direction. In a first attempt to solve this problem, the present inventors employed a circuit which sensed a change in direction of armature voltage, thus indicating that the operator intended to change direction of the truck. Upon sensing a change in direction in the armature voltage, previous control apparatus included a timing circuit which precluded reverse operation of the truck until a predetermined time had elapsed. This solution proved to be less than satisfactory because once the time delay had expired, the motor armature would be immediately subjected to maximum decelerating torque, which would result in an abrupt change similar to the undesired high torque problem. Another drawback to this solution was that when the truck was shifted from reverse to forward, the timing circuit prevented suitable acceleration of the truck.